The number of cases of the deadly bird flu virus is increasing around the world as scientists struggle to combat the disease that is now threatening to jump species and infect humans. The news comes as Britain confirmed its first ever case of H5N1 in a farm in Suffolk. More than 160,000 birds will now be slaughtered as the country's farming industry goes on high alert for more outbreaks.

As the authorities responded to the outbreak in Suffolk, local residents questioned why it had taken some 48 hours after the first chicks died last Tuesday for the government's Department of Food and Rural Affairs, Defra, to be informed. But there was also wider concern. Japan and Nigeria have reported a series of cases of the lethal virus, and China, Egypt, Russia, South Korea and Vietnam have also revealed outbreaks in birds and in humans in the past two months. Indonesia began a mass bird cull in the capital city of Jakarta last Wednesday. At least 164 people are now known to have been killed by the disease across the globe.

The rapid spread of avian flu can be traced to the fact that the H5N1 virus is mutating as it spreads through birds and a variety of mammals including cats, tigers and pigs, said Robert Webster, a researcher at St Jude Children's Research Hospital, in Memphis, Tennessee.

Last night British health officials insisted the risk to the public was still negligible. It set in motion a series of emergency measures aimed at halting the spread of the disease among poultry in Britain. All the turkeys on the affected farm at Holton near Lowestoft, the largest poultry centre run by the breeder Bernard Matthews, are now being culled. Yesterday, piles of slaughtered birds were being scooped into open tractor trailers before being removed for incineration.

An exclusion zone of three-kilometres (nearly two miles) has been set up around the farm. All poultry in the area are to be kept indoors and tested for the H5N1 virus. A further 10km zone has been established in which all movements of poultry are banned, and last night another 2,090sq km restriction zone was created to isolate poultry from wild birds. Other measures will include a ban on bird shows, poultry markets and pigeon racing. This follows a decision by Defra to revoke the national general licence on bird gatherings.

Last night, the National Farmers' Union president, Peter Kendall, said his organisation had warned members to be vigilant. A spokesman for Matthews said his organisation could confirm there had been a case of H5N1 avian influenza at its Holton site. But he added: 'It is important to stress that there is no risk to consumers.'

Dr Andrew Landeg, the government's deputy chief veterinary officer, revealed he had called in ornithologists in a bid to understand how wild fowl - now thought to be the most likely source of the Suffolk outbreak - might have carried the virus into one of the 22 turkey breeding sheds at the farm. Officials also needed to know what other areas of the country might now be at risk of outbreaks.

Last night, it was revealed that Defra had identified several areas in Britain considered to be at particular risk of picking up H5N1 from infected wild fowl. The farm at Horton was one of them.

Avian flu expert Dr Colin Butter of the Institute of Animal Health said it was now crucial that veterinary scientists discover whether the farm was the first place this strain of flu had emerged. It could be that the outbreak was an 'unhappy chance event', or it could indicate a significant level of the disease in wild birds indigenous to the UK, which in turn would make the virus harder to stamp out. Exclusion zones and culling would not be sufficient to contain the virus, Butter added.

However, Landeg yesterday ruled out the alternative - the introduction of vaccinations for poultry, a measure already introduced in Holland - as being ineffective.

As a result, farmers with free-range flocks may face serious threats to their livelihood. However, one organic poultry producer said moves to house birds permanently indoors would be counter-productive. 'My birds are resistant to the range of diseases that affect conventional flocks because their immune systems have been boosted through living outdoors. I accept that as a new strain of bird flu, special measures may be required in the short-term, but these should not be at the expense of the principles of good husbandry.'

However, Landeg yesterday insisted that the disease could be contained. 'There's a good hope, if this is not the first case, we will be able to contain this disease.'

It was revealed last night that five people in Indonesia have died from the virus since the beginning of the year while Japan yesterday confirmed its fourth outbreak of H5N1 bird flu this year and Thailand has reported three outbreaks during the past month.